1. Field of the Invention
This invention includes aspects of lens design for use in microscopes and fiberoptics applications, among other uses, single lens microscope lens optimization processes, single lens microscope lens designs, single lens microscope structures, single lens microscope focusing structures, slide holding and moving structures, slide position locking structures, multiple lens single lens microscope designs, illumination numerical aperture control mechanisms and processes for single lens microscopes, depth of field control mechanisms and processes for single lens microscopes, LED based illumination techniques for single lens microscopes, and photomicrography techniques for single lens microscopes.
Prominent structures, mechanisms, and techniques of the invention include hand-held portable single lens microscopes and their optical systems, designed to facilitate ease of use, to enhance safety, and to provide superior quality images over previous single lens microscopes.
2. Discussion of the Prior Art
Hand-held single lens microscopes are known in the prior art. The single lens microscope has existed in rather crude form since before the time of the great English scientist Robert Hooke, c.1640. Hooke seems to have been the first to have described the single lens microscope in print. In 1667, Hooke published his groundbreaking book Micrographia, stating in the preface:                If one of these lenses [a thin fiber of glass melted to form a small bead and polished with jeweler's polish] be fixed with a little wax against a needle hole pricked through a thin plate of Brass, Lead, Pewter or any other metal, and an object be placed very near be looked at through it, it will both magnify and make some objects more distinct than any of the large microscopes.        
Since the compound microscopes of Hooke's time were very crude, even a crudely made single lens microscope would produce superior results. It was apparently from this account by Hooke that Leeuwenhoek learned how to make single lens microscopes.
Antony van Leeuwenhoek, of Delft, Holland, refined the early single lens microscope in the period from 1668 to 1715 by improving the spherical form of the lens and by reducing its size to attain higher magnifications. His microscopes were created before the advent of the microscope slide, so their design was not suited to modem application. Since Leeuwenhoek never documented his lens design methods, the only information we have regarding his lenses has come from recent examination and testing of the nine remaining Leeuwenhoek microscopes by British scientist Brian J. Ford. (The Leeuwenhoek Legacy, Brian J. Ford, Biopress and Farrand Press, 1991, ISBN 185083016 9). Ford concludes that Leeuwenhoek's lenses were either formed by melting the tip of a glass fiber in a flame, thereby creating a droplet of glass with a shape approximating a sphere (referred to as a ‘fused lens’), or by forming a pointed glass bulb and melting the tip to form a droplet having a non-spherical unmatched pair of surfaces. This latter lens would take the form of some kind of double convex asphere, but the actual shape of the lens is largely uncontrolled, being formed by gravity, surface tension, and the physical details of the surrounding glass.
Most of Leeuwenhoek microscopes used hand-made glass lenses of roughly spherical figure held between thin metal plates, bearing crudely fashioned apertures, and attached to a screw mechanism designed to hold unmounted specimens. The design of a Leeuwenhoek microscope is impractical for use with modem microscope slides, and the lenses of Leeuwenhoek's manufacture, while remarkable for the time, were far from optimal. Furthermore, given the crudeness and irregularity of the lens apertures in the remaining Leeuwenhoek microscopes, there is no indication that Leeuwenhoek understood how to optimize the aperture of his single lens microscopes to provide the best image resolution.
During the period from 1810 to 1900 single lens microscopes were designed for use with microscope slides, but they never attained the image quality and magnifying power of Leeuwenhoek's instruments. They were awkward to use, having been patterned after the form of the compound microscope, and they soon fell into disfavor as scientists and naturalists turned to the compound microscope.
Hooke's instructions for making a single lens microscope simply refer to the use of a ‘needle hole’ of unspecified diameter. Neither Leeuwenhoek nor Ford have taught the need for lens aperture optimization, nor any method to do so. Three articles have been found that provide instructions for constructing a single lens microscope; none disclose an understanding of lens aperture optimization. (Build a Homemade Microscope”, C. L. Stong, The Amateur Scientist column, Scientific American, Jun. 1954, pg. 98; “To Make a van Leeuwenhoek Microscope Replica”, Alan Shinn, May 1996, at http://www.sirius.com/˜alshinn/Leeuwenhoekplans.html on the Internet; “Glass—Sphere Microscope”, Giorgio Carboni, Jan. 1996, at http://www.funsci.com/funsci.com/fun3en/usph/usph.htm on the Internet. In fact, the lens aperture suggestions provided in all three of the articles are seriously incorrect, yielding image resolutions with full aperture illumination that are very far from optimum.
Like Hooke, the Stong article simply refers to a ‘small’ hole to be used as the lens aperture. The Shinn article specifies a 1 mm aperture without regard for the diameter of a hand-made fused lens. Shinn has also stated, in private correspondence, that he prefers to use a 2 mm diameter BK-7 ball lens with a 1 mm aperture. Shinn's microscope is a replica of a Leeuwenhoek microscope, typically used with unmounted specimens, without a modem microscope cover glass over the specimen. The inventor has determined an optimized aperture diameters for a 2 mm diameter BK-7 ball lens to be 0.655 mm for 550 nm green light (the standard wavelength for computing resolution), providing a resolution of 1.3 microns, and 0.567 mm for polychromatic illumination (at the standard F, d, and C wavelengths) also providing a resolution of 1.3 microns. At Shinns specified aperture of 1 mm, aberrations limit the resolution to 15.0 microns and 11.2 microns for green and polychromatic illumination, respectively.
Carboni specifies a lens diameter of 1.2-2.5 mm and an aperture of 1.2 mm, used with microscope slides having cover glasses. The best performance for the specified aperture diameter would be with a 2.5 mm diameter lens. The resolution limit, at an illumination wavelength of 550 nm, of a 2.5 mm BK-7 ball lens with an optimized aperture of 0.780 mm, determined by application of the aperture optimizing methods described herein, is 2.0 microns. Carboni's specified 1.2 mm aperture limits the resolution of this lens to 10.6 microns.
Another hand-held single lens microscope is disclosed in U.S. Pat. No. 4,729,635. This design incorporates ‘lens beads’ of unspecified optical figure mounted in a clamp-like unit. Focusing is accomplished by pivoting one clamp component about the center of the other clamp component with finger pressure, thereby adjusting the distance between the lens and the slide.
U.S. Pat. No. 4,095,874 also discloses a hand-held single lens microscope which utilizes finger pressure to accomplish focusing by deflection of one cantilevered part bearing the spherical lens and a second cantilevered part holding the slide. The two parts are resiliently joined along a common edge. The two parts partially enclose the slide when the unit is in use, but the slide cannot be repositioned while it is being viewed. These microscopes lack optimized lens design, aperture optimization, a precise and stable focus device or mechanism, a versatile slide holding and moving device or mechanism, an external slide position control device or mechanism, a slide position locking device or mechanism, an aperture for illumination collimation control, and complete protection of the user from the sharp edges of the slide and from the pieces of a broken slide.
U.S. Pat. Nos. 4,737,016 and 5,844,714 disclose portable microscopes having a form reminiscent of Leeuwenhoek's microscopes. These designs incorporate low magnifying power lenses in a handle combined with various detachable devices for holding both unmounted specimens and standard microscope slides. Focus is accomplished by pivoting or sliding the specimen holding means closer or further from the lens. No specification is made regarding the type of lens to be used. These microscopes lack optimized lens design, aperture optimization, a precise and stable focus device or mechanism, a slide position locking device or mechanism, an aperture for illumination collimation control, and complete protection of the user from the sharp edges of the slide and from the pieces of a broken slide.
U.S. Pat. Nos. 5,572,370 and 5,267,087 disclose simple, low magnification microscopes intended for use in determining a woman's fertile periods. These instruments are not intended for use with standard specimen slides nor for general microscopic use. The overall form of these microscopes is cylindrical and internal illumination is provided by means of battery powered light sources. There is no provision to utilize ambient light for specimen illumination. These microscopes lack optimized lens design, aperture optimization, a precise and stable focus device or mechanism, a versatile slide holding and moving device or mechanism, an external slide position control device or mechanism, compatibility with standard microscope slides, a slide position locking device or mechanism, and an aperture for illumination numerical aperture control. The overall form, function, and intention of these microscopes are completely different from those of microscopes according to this invention.
U.S. Pat. No. 5,880,879 discloses a microscope objective lens system utilizing a diffractive optical element for chromatic aberration correction. This lens system is not a singlet, single element, lens, but rather a two lens system with an aperture stop in between. One example of a lens designed for microscopes according to the present invention is a singlet lens incorporating both aspheric and diffractive surfaces.
Inexpensive microscopes providing high image quality are needed for education, Third World medicine, scientific field research, and field medicine. These disciplines would benefit from application of single lens microscopes according to the present invention, since they can be low in cost, durable, and portable. Furthermore, single lens microscopes according to the present invention are suitable for use with conventional microscope slides, can provide high image quality at microbiologically and medically useful magnification using aperture optimized lenses, can provide protection from the sharp edges of intact or broken microscope slides, can provide a precise and stable focus device or mechanism, can provide a versatile slide holding and moving device or mechanism, can provide an external slide position control device or mechanism, can provide a slide position locking device or mechanism, can provide an aperture or a multiplicity of apertures for illumination numerical aperture control, can be simple and safe to use, and can use a variety of available light sources for illumination.
Definitions
    Aberration: A departure of an optical image-forming system from an ideal behavior.    Achromatic lens: A lens that brings two colors of light, typically red and blue, to substantially the same focal point    Aperture stop: The optical component that limits the size of the maximum cone of rays from an axial object point that can be processed by an entire optical system. Examples include the diaphragm of a camera and the iris of the human eye.    Apochromatic lens: A lens that brings three colors of light, typically red, green, and blue, to a substantially common focus and is often considered to also be spherical aberration corrected for two of these wavelengths.    Aspheric: An optical surface having a non-spherical form, generally described mathematically by a polynomial equation.    BK-7 glass: A common optical glass often used to make ball lenses. It is a desirable glass because it has low chromatic dispersion and thus lenses made from BK-7 glass have low chromatic aberration.    Center-to-center thickness error: Deviation of the actual thickness of a lens, as measured from the center of one optical surface to the center of the second optical surface, from the designed center-to-center thickness.    Chromatic aberration: The variation of focus with wavelength. Chromatic aberrations are caused by the fact that the refraction law determining the path of light through an optical system contains the refractive index n, which is a function of wavelength λ. Thus the image position and the magnification of an optical system are not necessarily the same for all wavelengths, nor are the aberrations the same for all wavelengths.    Compound microscope: A microscope that includes an objective lens system and an eyepiece lens system, wherein the objective lens system forms a magnified real image of a microscope specimen and the eyepiece lens system further magnifies this image, presenting a virtual image to the eye. The virtual image formation is thus indirect, being the result of a two step process with an intermediate real image. The total magnification of a compound microscope is the product of the magnification of the objective lens system and the eyepiece lens system. A compound microscope may incorporate as many as 30 lens elements in one objective/eyepiece pair.    Concave surface: A lens which has either one or both sides arched in toward the center; if both the lens is said to be double concave. A lens surface which is thicker at its edges than at its center, like )(.    Convex surface: Vaulted; arched; having a surface that curves outward, like the surface of a sphere; a lens surface which is thicker at its center than at its edges, like ( ).    Decentration error: Non-zero linear displacement between the axes of rotational symmetry of one two or more optical surfaces.    Depth of field: The distance limits along the optic axis above and below the focal plane of a lens at which the Modulation Transfer Function of the image drops below the threshold of visibility for a selected spatial frequency.    Diffractive optic (kinoform): A surface structure that can control the properties of light by means of diffraction. Diffractive optics are similar to holograms and diffraction gratings in that small grooves or lines across the optical surface impart a change in phase of the wavefront passing through the surface. Kinoforms can be incorporated into the refractive surfaces of lenses, providing an additional means for correcting aberrations.    Doublet: A lens incorporating two lens elements, usually made from different materials. The two lens elements may be physically separated, called air-spaced, so that they have four optical surfaces, or they may be cemented, such that they have two external optical surfaces and one internal optical surface.    Even asphere formula: Rotationally symmetric polynomial aspheric surfaces are commonly described by a polynomial expansion of the deviation from a spherical (or aspheric described by a conic) surface. The even asphere surface model uses only the even powers of the radial coordinate to describe the asphericity. The model uses the base radius of curvature and the conic constant. The surface sag is given by       Eq.  1:        z    =                            cr          2                          1          +                                    (                              1                -                                                      (                                          1                      +                      k                                        )                                    ⁢                                      c                    2                                    ⁢                                      r                    2                                                              )                                          +                        α          1                ⁢                  r          2                    +                        α          2                ⁢                  r          4                    +                        α          3                ⁢                  r          6                    +                        α          4                ⁢                  r          8                    +                        α          5                ⁢                  r          10                    +                        α          6                ⁢                  r          12                    +                        α          7                ⁢                  r          14                    +                        α          8                ⁢                  r          16                                where Z is the surface sag,        R is the base radius of curvature of the lens,        c=1/R,        k is the conic constant,        αI are coefficients on powers of r        and r is the radial lens position.            Field stop: The aperture in an imaging system that limits the field of view. This may be the same as the aperture stop or it may be different.    Flat field: A lens having a focal surface which is substantially planar is said to be a flat field lens.    Gradient index lens: A lens made from a material that has a non-uniform refractive index. Gradient index materials typically have refractive index which is a function of the radial position from the optic axis of the lens.    Huygens point spread function (PSF): The Huygens PSF computes the intensity of the diffraction image formed by the optical system of a single point source located a particular field position. One way of considering the effects of diffraction is to imagine each point on a wavefront as a perfect point source with an amplitude and phase. Each of these point sources radiates a spherical “wavelet”, sometimes called a “Huygens wavelet” after Huygens, who first proposed the model. The diffraction of the wavefront as it propagates through space is given by the interference, or complex sum, of all the spherical wavelets radiated.    Meniscus lens: A lens having one concave surface and one convex surface.    Like ( (.    Modulus of the optical transfer function (MTF): The ratio of the intensity modulation in the image to that in the object as a function of the frequency (cycles/mm) of the sine-wave object pattern. MTF is a measure of image contrast and sharpness of focus. A plot of MTF against spatial frequency v is an universally applicable measure of the performance of an image-forming system. It is defined as:             Eq.  2:        ⁢                             MTF      ⁡              (        v        )              =                            (                                    Max              1                        -                          Min              i                                )                /                  (                                    Max              i                        +                          Min              i                                )                                      (                                    Max              o                        -                          Min              o                                )                /                  (                                    Max              o                        +                          Min              o                                )                                Where:        Maxi=maximum image intensity        Mini=minimum image intensity        Maxo=maximum object intensity        Mino=minimum object intensity            Numerical aperture (NA): The index of refraction (of the medium in which the imaged object lies) times the sine of the half angle of the cone of illumination. Numerical aperture is used for systems that work at finite conjugates (such as microscope objectives).NA=n sin U  Eq. 3:Where:            n=refractive index of the medium containing the object        U=half angle of the cone of illumination            Planapo: Short for Plano Apochromat, a flat-field microscope objective lens that is chromatic aberration corrected for three colors and spherical aberration corrected for two colors.    Rayleigh criterion for resolution: The theoretically limiting resolution of an aberration-free optical system, defined to be the minimum distance between two objects at which they can be distinguished as separated images:             Eq.  4:        ⁢                       Z    =                  0.61        ∑            NA      At this limit of resolution, the two images are seen as being separated by a line which has an intensity equal to 74 percent of the peak intensity of the images. The Rayleigh criterion sets the theoretically best resolution for a lens of a given aperture. The performance of an actual lens will be degraded from this limit because of aberrations. The Rayleigh criterion is usually determined at a wavelength of ë=0.550 micron.    Resolution limit: The actual minimum distance between two objects at which they can be distinguished as separated images by a lens. This is different from the Rayleigh criterion because it accounts for the lens aberrations.    Real image: An image, formed by converging rays, which appears to be located at a point in space on the opposite side of a lens from the object. A real image can be focused on a screen without the need for reimaging by a second lens.    Single lens microscope: A microscope that utilizes a single optical system to directly form a magnified virtual image of a microscope specimen and to present it to the eye, or to directly form a real image onto an image recording device, such as photographic film. The lens system may consist of a singlet, containing one lens element, or it may consist of composite lens containing a multiplicity of lens elements, either air-spaced, cemented, or in combination. The single lens microscope is sometimes referred to as a ‘simple’ microscope, meaning that it has one lens.    Singlet: A lens incorporating two optical surfaces separated by a thickness of a single material.    Spatial frequency: The period of a black and white sine wave pattern, measured in terms of cycles per mm, which is used as a standard object for evaluating the Modulation Transfer Function performance of imaging systems.    Spheric: an optical surface having the form of a portion of a sphere.    Spherical aberration: (aperture aberration) can be defined as the variation of focus with aperture.    Strehl ratio: The ratio of the actual illumination intensity at the center of the focal spot produced by an imaging system divided by the center of the focal spot illumination intensity of an aberration-free imaging system. The Strehl ratio indicates how closely the performance of an optical system approaches the resolution limit imposed by the Rayleigh criterion. An imaging system having a Strehl ratio of 0.8 is considered to be diffraction limited.    Tilt error: Non-zero angular displacement between the planes normal to the axes of rotational symmetry of one two or more optical surfaces.    Triplet: A lens incorporating three lens elements, usually made from at least two different materials. The three lens elements may be physically separated, called air-spaced, so that they have six optical surfaces, or two elements may be cemented and one air-spaced, such that they have four external optical surfaces and one internal optical surface, or all three elements may be cemented in a stack, such that they have two external optical surfaces and two internal optical surfaces.    Virtual image: An image, formed by diverging rays, which appears to be located at a point in space on the same side of a lens as the object. A virtual image can be seen by imaging it with a positive lens, but it cannot be produced on a screen.
This invention includes a number of improvements on the single lens microscope. The design, utility, and optics of single lens microscope have not been significantly improved on since about 1700. Shortly after its initial development period, the single lens microscope was quickly replaced by the compound microscope, even though the image quality of the early single lens microscopes was superior to compound microscopes produced as late as 1850.
Short of the primitive technologies of survival, it can be argued that no development has had a greater impact on the improvement of humanity than the microscope. By means of the microscope bacteria and other pathogens were discovered, the germ theory of disease was developed, methods were developed to diagnose diseases, and methods were discovered to control pathogens. Microscopes are essential tools of modern science education, medical and veterinary training, nursing training, medical diagnosis, industrial inspection, and sciences of every persuasion.
In spite of their importance, the availability and use of high quality microscopes is limited by several factors. High quality compound microscopes are expensive, delicate, heavy instruments intended for use on a table or bench in a controlled, clean laboratory environment. Most modem compound microscopes also require AC electric power for their illumination system. Conventional compound microscopes used in schools often require dedicated facilities, including a special room equipped with dedicated work tables and electrical power drops.
The high cost, complexity, and fragility of high quality compound microscopes have limited their use in primary and secondary education. With the exception of high school biology laboratories, it is typical for schools in the United States to have just a few microscopes per school, usually with a single microscope in a classroom. Having only one microscope for a classroom of students creates a difficult classroom control problem for a teacher since she must attend to the student using the microscope while leaving the rest of the class unwatched. Compound microscopes are also difficult for children to learn to use, in part because the inverted images they produce are confusing; moving the slide in one direction results in the movement of the image in the opposite direction, making it nearly impossible to follow moving subjects.
In addition, teachers often hesitate to use conventional microscopes because of the risk of injury to the students from broken glass slides. Existing microscopes do not provide any protection to the student from the sharp edges of an intact microscope slide, nor do they provide protection from the razor-sharp edges of a broken slide. Furthermore, it is easy for a student to damage a costly microscope by running an objective lens into a slide, breaking the slide and scratching the lens. In developing nations the scarcity of microscopes is far worse.
A fundamental element of a modem education is missing for most students because of the high cost and limitations of conventional compound microscopes. The quality of education worldwide could be improved if a low cost, high resolution, durable and safe microscope was available.
An urgent need also exists for medical microscopes in developing nations; conventional laboratory microscopes are unsuitable for use in field medicine The conditions encountered by doctors in third-world refugee camps, unpowered rural villages, war zones, shantytowns, congested cities and slums are far from ideal. Even if they can afford a compound microscope suitable for medical work, it is not practical for doctors to carry such a heavy, delicate instrument to places that have no power. As a consequence, many fundamental medical diagnostic tests, such as blood cell counts, pap smears, tissue biopsies, and urinalysis, are not performed.
For example, in Africa there are two predominant strains of malaria; a less virulent strain that is treated with a low cost antimalarial drug, and a more virulent strain that requires treatment with a more expensive drug. The type of malaria infecting a person can be easily determined by microscopic examination of a blood smear, but microscopes are unavailable, so everyone gets treated with the lower cost antimalarial drug. The people with the less virulent strain survive, but those infected with the more virulent strain die. A simple blood smear examined under a suitable microscope could allow doctors to make the right choice to save these people.
It is clear that there is an urgent worldwide need for low cost, lightweight portable, safe, durable, high performance microscopes that do not require electrical power. Devices and microscopes according to the present invention can satisfy this need.